In recent years, techniques for producing water containing fine bubbles have been studied actively. For example, online catalog “Nanobubble Generator FZ1N-02” (May 23, 2011, IDEC, Corporation retrieved on Dec. 20, 2011, Internet <URL: http://www.idec.com/jpja/products/dldata/pdf_b/P1383-0.pdf>) discloses an apparatus for generating a lot of fine bubbles. Furthermore, Koichi TERASAKA and five others report in “Study on Analysis of Nanobubbles Generated by Gas-Liquid Mixture Shearing Method” (Proceedings of Annual Meeting 2011 of the Japanese Society for Multiphase Flow, Executive Committee of Annual Meeting 2011 (Kyoto) of the Japanese Society for Multiphase Flow, August 2011, pp. 424-425) that fine bubbles are stable in water.
There are various techniques for generating fine bubbles. As one example, in a fine bubble generating apparatus disclosed in Japanese Patent No. 4129290, a gas-liquid mixed fluid is swirled at high speed in a fluid swirling chamber so that the shearing force produced in the swirl flow reduces the sizes of bubbles.
Water containing fine bubbles has thus far been generated by mixing water and air mechanically. The number of fine bubbles generated by mechanical mixing is currently approximately in the range of one hundred million to one billion bubbles per milliliter (bubbles/mL). The number of fine bubbles per unit volume is increasing with advances in technology, but we cannot expect a dramatic increase in a short term. Meanwhile, increasing the density of fine bubbles is important for research on applications of liquid that contains fine bubbles and to increase the efficiency of handling liquid that contains fine bubbles.